1. Field of the Invention
The present invention relates to aging of a grating built in an optical waveguide, and more particularly to aging of a grating used as a filter, multi/demultiplexer, dispersion-compensator, and the like in an optical fiber network. The present invention also relates to a temperature sensor including an optical waveguide grating as a sensing section.
2. Related Background Art
An optical waveguide type of grating, which is typified by an optical fiber grating, is a region in an optical waveguide such as an optical fiber (mostly in its core portion) in which a periodic change of refractive index along the longitudinal direction of the waveguide occurs. The region where the refractive index changes can transmit or reflect propagated light depending on its wavelength. In particular, a Bragg grating generates reflected light with a narrow wavelength band centered on its Bragg wavelength. The optical waveguide grating is applied to various optical elements such as filters, multi/demultiplexers, dispersion-compensators, and the like.
FIG. 1 is a view showing a typical method for producing an optical waveguide grating. As shown in FIG. 1, a grating 20 is often formed by a method comprising a step of preparing a silica-based optical fiber 10 in which GeO2 (germanium dioxide) is added to at least its core region; a step of irradiating this optical fiber 10 with an interference fringe formed by light rays 30 having a predetermined wavelength; and a step of generating a change in refractive index dependent on the optical energy intensity distribution of this interference fringe. Since the optical fiber 10 is usually coated with a plastic layer (not shown), a part of the coating is removed, and thus exposed part of the optical fiber 10 is irradiated with the light rays 30. It has been considered that the irradiation with a certain wavelength of light generates Ge-defects in the GeO2-doped portion in the silica-based optical waveguide, thereby causing the change in refractive index. In FIG. 1, numeral 22 indicates parts where a larger amount of increase in refractive index is induced upon the irradiation, whereas numeral 24 indicates parts where a smaller amount of increase in refractive index is induced. The grating 20 may be considered to be a region where the parts 22 and 24 are alternatively and periodically disposed along the longitudinal direction of the optical fiber 10.
An optical waveguide grating may be used as a temperature sensor also. The temperature sensor comprises an optical waveguide grating as a sensing section, and measures temperature utilizing the temperature dependence of the Bragg wavelength. More particularly, in the measurement of temperature, the sensor measures the Bragg wavelength and compares the measured value with the temperature dependence of the Bragg wavelength previously measured to determine the temperature.
As is previously known, the characteristics of an optical waveguide grating change over time because the number of Ge-defects generated by the irradiation of light changes over time. This has been known as aged deterioration of an optical waveguide grating. With respect to a Bragg grating, the Bragg wavelength at any temperature changes (usually decreases) over time. It means that the operating characteristics of a temperature sensor comprising an optical waveguide Bragg grating as its sensing section change over time. For example, if such a temporal change is relatively rapid, different Bragg wavelengths are measured for the same temperature one month and three months after beginning to use the temperature sensor, and thus different temperatures will be determined at different points in time. In view of the foregoing, there have been proposed techniques which performs accelerated aging for an optical waveguide grating immediately after its manufacture to sufficiently suppress its aged deterioration upon operation in the market. Examples of such techniques are disclosed in U.S. Pat. Nos. 5,287,427 and 5,620,496 which are incorporated herein by reference.
In the technique disclosed in U.S. Pat. No. 5,620,496, normalized refractive index difference xcex7 is supposed to be represented by the following relational expression:                     η        =                  1                      1            +                          C              ·                              t                α                                                                        (        1        )            
where t represents time, and C and xcex1 are functions of temperature. The normalized refractive index difference xcex7 is a value of the refractive index difference of a grating when time t has elapsed from a predetermined point of time (i.e., reference time) after formation of the grating, and this value is normalized with respect to the refractive index difference of the grating at this point of time. Namely, xcex7=(refractive index difference at t after the reference time)/(refractive index difference at the reference time). In the technique disclosed in the above patent, the time immediately after formation of a grating is adopted as the reference time. The refractive index difference refers to the difference between the maximum and minimum values. of the refractive index in a grating.
In the conventional techniques, from the fact that xcex7 changes more rapidly as temperature is higher, the optical fiber grating is heat-treated in an environment with temperature higher than its operating temperature to perform the accelerated aging, in order to suppress the deterioration upon its operation.
Having studied the conventional techniques-mentioned above, the inventors have found the following problems. Namely, in the above-mentioned conventional techniques, since expression (1) which represents the temporal change in normalized refractive index difference xcex7 has the relatively complicated form and the two parameters of C and xcex1 depend on temperature, it is difficult to determine the temperature and time of the heat treatment for the aging. In effect, the above-mentioned patents do not fully disclose such conditions of the aging.
It is an object of the present invention to provide a method by which a condition of aging may be determined more easily.
More specifically, the method in accordance with the present invention comprises a step of setting the aged deterioration curve of an optical waveguide grating as a form of Cxc2x7txe2x88x92xcex1, where t represents time, and C and xcex1 represent parameters; and a step of determining a condition of aging according to said aged deterioration curve. The aging condition can be determined more easily because the form of the aged deterioration curve that is proportional to txe2x88x92xcex1 is simpler than that in the prior art. Parameter a may be represented as follows:
xcex1=xcex10xc2x7exp(xe2x88x92Excex1/T)
where xcex10 and Excex1 are constants, and T is absolute temperature. Since these expressions can represent aged deterioration of an optical waveguide grating with sufficient accuracy, these expressions may be used to determine an aging condition adequately.
Parameter C may be represented as follows:
C=xcfx84xcex1=[xcfx840xc2x7exp(xe2x88x92Excfx84/T)]xcex1
where xcfx840 and Excfx84 are constants, and T is absolute temperature. This expression provides a good representation of aged deterioration of an optical waveguide grating at high temperature. Therefore, the expression may be used to determine an aging condition for an optical waveguide grating adequately even if the grating is used in relatively high temperature environment.
In one embodiment, parameter C may be regarded as a constant. In this case, the expression of the aged deterioration curve may include only one parameter dependent on temperature, i.e., xcex1, and thus the aging condition can be determined still more easily.
Further, in one embodiment, the value xcex71 of the normalized refractive index difference at the completion of the aging may be determined as the aging condition. The temperature T1 and time t1 of the heat treatment for the aging can be determined from the value xcex71.
Another aspect of the present invention includes a method for making an optical waveguide with a grating which is designed to suppress its deterioration within a predetermined tolerance when the grating is used for an operating time of t3 at an operating temperature of T2. This method comprises a step of forming a grating in a region in an optical waveguide; and a step of aging the grating under an aging condition determined by the above-mentioned determining method. In one embodiment, the aging may be effected by heat-treating the grating until the normalized refractive index difference xcex7 of the grating reaches the above value xcex71.
Another object of the invention is to provide a temperature sensor with operational stability over a long period of time by suppressing temporal changes in its operating characteristics.
A temperature sensor in accordance with the invention has an operating temperature range, a guaranteed operating time and a temperature resolution, and comprises an optical waveguide grating as a sensing section. The grating has been subjected to accelerated aging under a predetermined condition. The condition of the aging is determined to provide a displacement of a measured temperature value due to aged deterioration of the grating that is no greater than the temperature resolution when the sensor is used for the guaranteed operating time at the maximum temperature in the operating temperature range. In the temperature sensor in accordance with the invention, since the optical waveguide grating subjected to accelerated aging under such a condition is used as the sensing section, variations in the operating characteristics due to the aged deterioration of the grating may be sufficiently suppressed.
In one embodiment, the aging condition may be determined using the aged deterioration curve of the grating represented as said form of Cxc2x7txe2x88x92xcex1, where t represents time, and C and xcex1 represent parameters.
The optical waveguide in the sensor in accordance with the invention may have the surface subjected to etching after the accelerated aging. Since the etching may eliminate a scratch on the surface of the waveguide, a risk of breakage of the waveguide due to thermal deformation may be decreased when the sensor is placed in an environment with varying temperature.
The optical waveguide in the sensor may have a heat-resisting coating as its surface. The temperature sensor with such a waveguide may function well at relatively high temperatures.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.